The present invention relates to a multi-layer golf ball and methods for forming a portion thereof including a core having a center with at least one center layer, a mantle having at least one mantle layer including an amount of reinforcing polymer component and a resilient polymer component disposed concentrically adjacent the center, and at least one cover layer disposed concentrically adjacent the core, wherein the mantle or at least one layer of the mantle is sufficiently rigid to inhibit the resilient polymer component from substantially altering shape prior to crosslinking. The invention also relates to the polymeric composition used in forming the mantle.
Multi-layer golf balls contain a core, which may include one or more layers of solid material or one or more layers of solid material encompassing a liquid therein, and a cover. Optionally, an elastic winding may also be used to form a layer surrounding the center to provide certain playing characteristics. Such balls are known as xe2x80x9cwoundxe2x80x9d balls. The multi-layer golf balls discussed herein include a core and a cover. The terms xe2x80x9ccorexe2x80x9d or xe2x80x9cball core,xe2x80x9d as used herein, include a center having one or more layers and a mantle formed of one or more layers. The terms xe2x80x9ccenterxe2x80x9d or xe2x80x9cball center,xe2x80x9d as used herein, include a solid and/or liquid mass around which at least a mantle and cover are placed. The mantle is disposed between the center and the cover, typically in concentric fashion, with the cover being the outermost portion of the ball.
A variety of golf ball compositions are known and used in various methods of manufacture. Unfortunately, these compositions and methods tend to produce balls that do not consistently achieve a symmetrical core. See, for example, the discussion in co-pending application Ser. No. 08/943,932 by J. DALTON et al., which illustrates the poor centering that occurs in conventionally formed golf balls. This co-pending application is expressly incorporated herein by reference thereto for this purpose. Multi-layer ball production has been plagued by center portions that become off-centered during the manufacture of such balls. Off-center golf balls are a hindrance to many players, particularly those able to achieve great control using a symmetrical ball. This lack of symmetry is now believed to be caused, at least in part, by the materials and methods conventionally used in forming multi-layer golf balls. A number of these conventional multi-layer ball compositions are discussed below.
U.S. Pat. No. 4,781,383 discloses a solid three-piece golf ball made by covering a core, which has inner and outer layers, with a shell. The outer layer of cis-1,4-polybutadiene, zinc diacrylate, and zinc oxide is prepared by using a metal mold to prepare two hemispherical premolded products, which are used to cover the previously molded inner layer of the core. The outer layer is then cured around the inner layer by heating the entire core before adding the shell.
U.S. Pat. No. 4,919,434 discloses a two-piece golf ball having a solid core of more than 40% cis-1,4-polybutadiene and a cover having an inner layer of 0.1 to 2 mm thickness and an outer layer of 0.1 to 1.5 mm thickness. The inner layer is a thermoplastic resin, such as an ionomer, polyester elastomer, polyamide elastomer, thermoplastic urethane elastomer, propylene-butadiene copolymer, 1,2-polybutadiene, polybutene-1, and styrene-butadiene block copolymer, either individually or in combination.
U.S. Pat. No. 5,150,905 discloses a rubber composition usable in golf balls having at least one natural or synthetic rubber component, inorganic fibers subjected to surface treatment, and a non-sulfur type vulcanizing agent. The rubber may include known additives, such as organic modifiers of various resins like cumarone-indene, phenol, polystyrene, acrylic, polyamide, epoxy, urethane, polyolefin, and similar resins. The rubber may also include long fiber reinforcing material, such as fibers of glass, carbon, metal, quartz, ceramic, nylon, vinyl, polyester, aromatic polyamide, polyimide, and aromatic polyether amide.
U.S. Pat. No. 5,253,871 discloses a three-part golf ball including an elastomer core, an intermediate layer of a thermoplastic material containing at least 10%, preferably at least 35%, of ether block copolymer, and a thermoplastic envelope. The other copolymer of the intermediate layer is disclosed to be one or more ionomers.
U.S. Pat. No. 5,314,187 discloses a golf ball having a core, as well as a cover having an inner layer of a cut-resistant material such as an ionomer resin and an outer layer of natural or synthetic balata and one or more thermally crosslinkable elastomeric polymers.
U.S. Pat. No. 5,439,227 discloses a multi-piece solid golf ball having a solid core with an inner layer of a rubber and an outer layer of 100-50 wt % of a polyether ester type thermoplastic elastomer having a Tg of up to xe2x88x9225xc2x0 C. and 0-50 wt % of an ethylene-(meth)acrylate copolymer ionomer, and a cover of ethylene-(meth)acrylate copolymer ionomer.
U.S. Pat. Nos. 5,553,852 and 5,556,098 disclose a three-piece solid golf ball with a conventional rubber center core, an intermediate layer of thermoplastic elastomer or thermoplastic elastomer and ionomer resin mixture, and a cover typically of an ionomer resin, each portion having a particular hardness and thickness.
U.S. Pat. No. 5,601,502 discloses a three-piece solid golf ball including a core of a center having an xcex1,xcex2-unsaturated carboxylic acid metallic salt in an amount of 13 to 28 parts by weight based on 100 parts by weight of base rubber and an outer shell having an xcex1,xcex2-unsaturated carboxylic acid metallic salt in an amount of 28 to 35 parts by weight based on 100 parts by weight of base rubber. The base rubber preferably has a cis-1,4 structure of 40% or more, particularly 85% or more.
U.S. Pat. No. 5,681,898 discloses a golf ball having a solid core and a cover, with an intermediate layer including a first component of an uncrosslinked blend of n-butyl acrylate and ethylene methacrylic acid copolymer, which is sold under the name NUCREL, and a second component of a vulcanizate formed from polybutadiene and a peroxide curing agent. The vulcanizate is ground to a fine powder and then conventionally mixed with pellets of the NUCREL and melted for injection molding.
U.S. Pat. No. 5,683,312 discloses a golf ball having a fluid mass at the center, a first non-wound mantle layer of a thermoset rubber material, thermoplastic elastomeric material and plastic, a second non-wound mantle layer of a thermoset rubber material or thermoplastic elastomeric material, and a cover.
U.S. Pat. No. 5,688,191 discloses a multi-layer golf ball having a core with one or more layers, at least one cover layer, and one or more mantle layers disposed therebetween, wherein the mantle layer includes dynamically vulcanized thermoplastic elastomer, functionalized styrene-butadiene elastomer, thermoplastic polyurethane, metallocene polymer or blends thereof, and thermoset materials.
It is desirable to use thermoset material-containing hemispherical shells to form one or more mantle layers about a golf ball center, although this often results in poor centering of the mantle and other difficulties because thermoset materials are difficult to work with before they have been crosslinked. The polymers typically used in such shells tend to have a memory that urges the polymer back to its earlier or original shape, which necessitates rapid compression molding to crosslink the polymer as soon as the shells are formed. Hemispherical shells are also prone to trapping air in the apex of the mold cavities in which they are placed to assemble golf ball cores. Hemispherical shells also do not tend to readily fit within mold cavities having off-center parting lines, which also causes problems due to poor centering when forming golf ball cores.
There is thus a need for an improved composition and method for manufacturing golf balls that avoids the disadvantages present when using thermoset material-containing hemispherical shells to form one or more mantle layers about a center. A new composition for one or more layers of a golf ball mantle, and a method for manufacturing a portion of a golf ball core using ellipsoidal shells around a center, advantageously improves the symmetrical formation of the core in golf balls in accordance with the present invention. The proposed compositions facilitate injection molding of the uncrosslinked shells and permit automated assembly, which greatly reduces production costs.
The present invention relates to a method of forming at least a portion of a golf ball core by mixing a resilient polymer component, a free-radical initiator, a crosslinking agent, and a reinforcing polymer component to provide an uncrosslinked first mixture having a rigidity measured by a flexural modulus of greater than about 3.5 MPa, wherein the mixing occurs at a temperature greater than the melting temperature but sufficiently lower than the crosslinking temperature of the reinforcing polymer component so as to substantially inhibit initiation of crosslinking, forming the first mixture into a plurality of shells in a desired shape, wherein the reinforcing polymer component imparts sufficient rigidity to the shells to maintain the desired shape until the first mixture is crosslinked, providing a center, assembling at least two shells concentrically about the center to form a first mantle layer, wherein the first mantle layer and center together form the ball core, and applying sufficient heat and pressure to the core as it is constrained within a rigid cavity for a time sufficient to at least partially crosslink the first mixture in the shells, thereby curing at least a portion of the golf ball core. In a preferred embodiment, the first mixture is formed into ellipsoidal shells.
In one embodiment, the method includes selecting the resilient polymer component to have a molecular weight average of between about 50,000 to 1,000,000. In another embodiment, the reinforcing polymer component is generally selected to have a crystalline melting temperature between 35xc2x0 C. and 120xc2x0 C. In another embodiment, the first mixture is formed into a plurality of shells by injection molding. These shells are preferably ellipsoidal in shape. In yet another embodiment, the desired ellipsoidal shells are formed by compression molding the first mixture.
In a further embodiment, the ball core has a midpoint and the center of the core is disposed within about 0.5 mm from the midpoint. In another embodiment, the uncrosslinked first mixture used to form the mantle has a flexural modulus that is at least about 7 MPa prior to cure. In yet another embodiment, the loss tangent of the crosslinked mantle material first mixture is adjusted to less than about 0.15 at xe2x88x9260xc2x0 C. and less than about 0.05 at 30xc2x0 C. at 1 Hz and 1 percent strain. In another embodiment, the dynamic modulus, used herein to mean the tensile storage modulus (Exe2x80x2), of the crosslinked first mixture is greater than about 100 MPa at xe2x88x9260xc2x0 C. and greater than about 50 MPa at 30xc2x0 C., also at 1 Hz and 1 percent strain.
In another embodiment, the melting temperature and the crosslinking temperature are selected to differ by about 60xc2x0 C. to 160xc2x0 C. In yet another embodiment, the core is selected to include a center surrounded by elastic windings, a solid center, or a liquid center. In another embodiment, at least one additional layer is formed about the center prior to assembling the shells concentrically about the center, after assembling the shells concentrically about the center, or after heating the core. In a preferred embodiment, at least one additional layer is formed around the core after heating the core to provide a cover disposed concentrically about the golf ball core.
The invention also relates to elastomeric compositions including a resilient polymer component of at least one polybutadiene having a high molecular weight average and a 1,4-cis content of greater than about 50 weight percent, a free-radical initiator, and an amount of reinforcing polymer component having a sufficiently low viscosity at a mixing temperature to facilitate mixing of the reinforcing polymer component with the resilient polymer component and having a crystalline melting point sufficiently low to facilitate mixing while avoiding substantial crosslinking, wherein the uncrosslinked composition has a flexural modulus of greater than about 3.5 MPa.
In one embodiment, the composition further includes a crosslinking agent in an amount sufficient to increase crosslinking between the polymer components. The invention also relates to an ellipsoidal article formed of the composition.
In another embodiment, the resilient polymer component has a molecular weight from about 50,000 to 1,000,000. In a preferred embodiment, the molecular weight average of the resilient polymer component is from about 250,000 to 750,000.
In a further embodiment, the free-radical initiator is an organic peroxide. In another embodiment, the reinforcing polymer component includes a block copolymer ether/ester, an acrylic polyol, a transpolyisoprene, a transpolybutadiene, a 1,2-polybutadiene, an ethylene-vinyl acetate copolymer, or a cyclooctene. In yet another embodiment, the composition also includes a crosslinking agent of a metallic salt selected from the group of an unsaturated fatty acid, a monocarboxylic acid, and mixtures thereof. In another embodiment, the reinforcing polymer component is present in an amount of about 1 to 40 weight percent of the resilient and reinforcing polymer components.
The invention also relates to a multi-layer golf ball having a core including a center, a mantle having at least one layer, the layer having a blend of a reinforcing polymer component and a resilient polymer component crosslinked and disposed concentrically adjacent the center, and at least one cover layer disposed concentrically adjacent the mantle and outwardly thereof, wherein the uncrosslinked mantle layer is sufficiently rigid to inhibit the resilient polymer component from substantially altering shape prior to crosslinking.
In another embodiment, the resilient polymer component and the core each include polybutadiene, natural rubber, polyisoprene, styrene-butadiene, or styrene-propylene-diene rubber, or mixtures thereof. In a preferred embodiment, the resilient polymer component is 1,4-cis-polybutadiene having a 1,4-cis content of greater than about 50 weight percent. In a preferred embodiment, the 1,4-cis content is greater than about 90 weight percent.
In still another embodiment, the amount of resilient polymer component is between about 60 to 99 weight percent of the polymer components. In a preferred embodiment, the amount of resilient polymer component present is from about 75 to 90 weight percent of the polymer components. In another embodiment, the golf ball core further includes at least one of a filler, a free-radical initiator, or a crosslinking agent. In one preferred embodiment, filler is present and includes masterbatch red, zinc oxide, tin oxide, barium sulfate, zinc sulfate, calcium carbonate, barium carbonate, clay, tungsten, tungsten carbide, a silica, or trimethyl-tripropane, or mixtures thereof, present in an amount from about 0.5 to 50 weight percent of the mantle. In another preferred embodiment, the free-radical initiator is an organic peroxide. In another preferred embodiment, the crosslinking agent includes a metallic salt selected from the group of an unsaturated fatty acid, a monocarboxylic acid, and mixtures thereof. In another embodiment, the uncrosslinked mantle layer has a flexural modulus of greater than about 3.5 MPa.